OBJECTIVE: to characterize the onset of messenger RNA (mRNA) and ribosomal RNA (rRNA) synthesis and the expression of fibrillarin during preimplantation development in in vitro fertilized macaque embryos using sensitive autoradiographic and immunocytochemical techniques. RESULTS In in in vitro fertilized macaque embryos developing in vitro, nuclear transcription of mRNA is initiated at the 2-cell stage and increases markedly at the 8-cell stage, with the onset of nucleolar synthesis of rRNA transcription and the expression of the nucleolar protein, fibrillarin. Fibrillarin was localized to the nucleolar periphery at all stages of development and was not expressed in the absence of rRNA synthesis. These results have important implications in understanding the molecular causes of developmental failure in primate embryos and in the production of cloned and/or transgenic primate embryos by nuclear transfer technologies. FUTURE DIRECTIONS We plan to extend these studies, as well as studies involving expression of stage-specific genes, to embryos derived from in vitro matured oocytes and to reconstructed embryos produced by transfer of nuclei from in in vitro fertilized embryos and from fetal and adult fibroblasts, in order to study impairments in genome activation and nuclear reprogramming. KEY WORDS messenger RNA, ribosomal RNA, fibrillarin, autoradiography, immunocytochemistry